Composer support

This pull request turns Baikal into a modern PHP project, by adding composer support :)

This means that when [composer is installed](http://getcomposer.org/), installing baikal is a matter of hitting:

```
composer create-project jeromeschneider/baikal
```

In addition, the central composer autoloader is now used, which should simplify your bootstrapping a bit, and SabreDAV is no longer part of the source.. it's simply installed as a dependency.

After checking out your project, simply hit:

```
composer install
```

To fetch all dependencies and generate the autoloader, or

```
composer update
```

To update to the latest SabreDAV 1.8.x version.


I hope you like it! If you do, you should register baikal here: http://packagist.org